Conventionally, a rotation angle sensing device for detecting a rotation angle of a rotating body is used for various purposes. One such rotation angle sensing device is provided with a magnet, which is fixed to rotate with a rotating body, and a magnetic sensor element for detecting a change in intensity of a magnetic field in association with the rotation of the magnet. In such rotation angle sensing device, the magnetic sensor element outputs a signal indicating a relative positional relationship between the rotating body and the magnetic sensor element.
A known conventional rotation angle sensing device, as shown in FIG. 23A and FIG. 23B, includes a magnet 200 that is disk-like and is supported by and fixed to a shaft S (rotary shaft) to allow a first surface 201 and a second surface 202 of the magnet 200 to be orthogonal to the shaft S (rotary shaft). A magnetic sensor element (Hall element) 300 is arranged immediately beneath an outer circumference of the second surface 202 of the magnet 200 and in a circumferential direction about the shaft S (rotary shaft) (see Patent Literature 1).
In the rotation angle sensing device, because shaft wobble occurs and causes the shaft S (rotary shaft) to minutely move, the magnet 200 supported by and fixed to the shaft S (rotary shaft) minutely moves in the radial direction. In the meantime, the magnetic sensor elements (Hall elements) 300 are arranged to measure magnetic flux density in a direction parallel to the shaft S (rotary shaft) at a corner portion of the outer circumference of the magnet 200. Consequently, a measured value for the magnetic flux density that is measured by the magnetic sensor elements (Hall elements) 300 varies greatly in association with the minute movement of the magnet 200, and an error in measurement of a rotation angle is accentuated.
Conventionally, as shown in FIG. 24A and FIG. 24B, a rotation angle sensing device that is provided with a magnet 210, which is supported by and fixed to the shaft S (rotary shaft), a first surface 211 and a second surface 212 opposed the first surface 211, and magnetic sensor elements (Hall elements) 310 arranged immediately beneath an outer circumference of the magnet 210. The magnet 210 has a chamfering part 213 (inclined surface) formed by eliminating corners of the outer circumference at the second surface 212 side. A portion of a detector plane in the magnetic sensor elements (Hall elements) 310 is positioned immediately beneath the chamfering part 213 (inclined surface) and a remainder is arranged to be positioned outside the outer circumference of the magnet 210 (see Patent Literature 2).